1. INTRODUCTION

In the X-ray band, as in the radio region, one studies virtually every
type of astronomical system. Both channels are good indicators of
non-thermal activity, and of the beginnings and ends of the lifecycle
of stars and massive black holes. In the early days of X-ray
astronomy, when locations of sources were no more precise than the
order of a square degree, the coincidence of a strong radio source in
the region served as an argument for the X-ray identification, for
example, M87
(Bradt 1967)
and 3C 273
(Bowyer 1970).

As a theme for this review, I will concentrate on systems where the
X-ray and radio observations are telling us about the outflow of
material and energy. This is in contrast to what might be called
"classical" X-ray astronomy, which studied accretion processes.
Infall of gas not only explained the energy source, but also led
directly to the calculations of X-ray emission from disks in galactic
neutron star, black hole, and white dwarf binaries. Accretion onto
supermassive black holes was inferred to power quasars and Active
Galactic Nuclei (AGN). In these cases the X-ray and non-thermal
optical continuum are most closely related. If accretion disks were
the only setting for X-ray emission, there might be few X-ray and
radio connections, as the peak radiation frequency corresponds to the
temperature in the innermost disk, and therefore does not span the
very broad range from
109 Hz
radio emission to
1018 Hz
X-ray emission.

Figure 1 provides examples of systems representing
the three topics I will discuss: pulsars and supernova remnants; cooling
flows in clusters of galaxies; and jets in quasars and radio galaxies.

In clusters of galaxies, the X-ray emission from the hot gas filling
the volume between the galaxies was shown to have temperatures
consistent with the gravitational potential of the cluster. In a
substantial fraction of clusters, the gas was observed to be
sufficiently dense that it would cool in much less than a Hubble time,
and it was interpreted that massive cooling flows involving hundreds of
solar mass per year were condensing onto the cluster centers
(Arnaud (1988),
Fabian (1994)).
This created a great puzzle, as neither the destination of the cooling gas,
nor great quantities of gas at temperatures less than 1 to 2 keV have
been found. It now appears that powerful radio sources in cD galaxies
in the cluster cores
(Burns (1990))
provide the energy to offset the cooling.

In contrast to accretion, a major theme of radio astronomy has been
the origin of cosmic rays, the acceleration of particles to ultra high
energies, and the transport of energy in jets to distances of pc to Mpc
away from the nuclei of active galaxies. Radio astronomy has been primarily
an imaging rather than spectroscopic science (with some important
exceptions which we have heard at this symposium). In this article I
therefore emphasize X-ray imaging. For decades radio observations have
studied detailed structure in supernova remnants, emission from
cluster of galaxies and sources in clusters, and jets in active
galaxies. With the half-arcsecond X-ray imaging now available from the
Chandra X-ray Observatory, we can finally compare X-ray with
GHz radio observations on the same angular scales.